Using Femtosecond Lasers in Multiplex Spectroscopy

EuroPhotonicsMar 2008
When conducting broadband molecular spectroscopy, researchers should use a high-brightness light source to obtain optimum results. Spectrometers used for analysis in the mid-infrared region, especially, should have high sensitivity, resolution and accuracy, because many molecules have characteristic vibration-rotation transitions in this domain. For detecting multiple species, such devices also should permit simultaneous detection.

Researchers from the Institut für Photonik in Vienna, Austria, the Norges Teknisk Naturvitenskapelige Universitet in Trondheim, Norway, and the Centre National de la Recherche Scientifique-Laboratoire de Photophysique Moléculaire in Orsay, France, have designed a multiplex spectroscopy device that fulfills these criteria. Rather than a tungsten lamp or other blackbody light source, their assembly incorporates a femtosecond laser.

A chromium-doped zinc selenide crystal in a copper heat sink forms part of a mode-locked laser used for gas analysis. Courtesy of Nathalie Picque, Centre National de la Recherche Scientifique.The scientists built their setup around a chromium-doped zinc selenide crystal. An uncoated lens focused light from an erbium-doped fibre laser onto the crystal to provide optical pumping. A chirped mirror focused the beam onto a semiconductor saturable absorber mirror, which allowed self-starting mode locking. Along with an output coupler, two dichroic focusing mirrors and a YAG plate for dispersion compensation, the components formed an X-fold cavity around the crystal. To eliminate water vapor absorption lines, the researchers housed the components in a nitrogen-filled enclosure with transparent entrance and exit windows. Once assembled, the mid-infrared oscillator provided pulses with a 130-fs duration.

Beyond the exit window, the scientists placed a 70-cm-long absorption cell filled with the gas of interest. After leaving the cell, the laser beam passed through an attenuator to a Bruker Fourier-transform spectrometer with an indium arsenide detector and fluorine beamsplitter for analysis.

Filling the cell with acetylene and ammonia, the researchers then recorded rovibrational spectra at 2.4 μm for 13-s periods. Acetylene levels as low as 22 parts per billion by volume (ppbv) were detected with a signal-to-noise ratio of 3800 and a spectral domain of 220 nm. The group detected ammonia to around 160 ppbv. For hydrogen fluoride, this would translate to a 0.2 ppbv detection level. In all cases, resolution was 3.6 GHz. By comparison, spectra recorded using a tungsten lamp-based system required a 300-fold increase in measuring time to obtain an identical signal-to-noise ratio.

The scientists note that their system may yield improved sensitivity if it were fitted with a high finesse cavity or multipass cell. Taking advantage of the laser’s comb structure could allow high-frequency synchronous detection, while the addition of a higher-resolution interferometer could improve the resolution of the device.